Photo scanner and image forming device

ABSTRACT

A first reflecting optical element ( 7 ) and a second reflecting optical element ( 51 ) are provided, and the second reflecting optical element ( 51 ) is disposed so as to reflect incident light to be incident on the first reflecting optical element ( 7 ) and reflected light from the first reflecting optical element, and thus a small-sized optical scanner can be obtained.

TECHNICAL FIELD

The present invention relates to an optical scanner used in a laser beamprinter, a laser facsimile, a digital copier or the like and an imageforming apparatus.

BACKGROUND ART

Most types of optical scanners used in laser beam printers and the likehave a configuration including a semiconductor laser as a light source,a first image forming optical system that linearly focuses a light beamfrom the light source on an optical deflector so as to compensate forthe tilt of a deflection surface of the optical deflector, a polygonmirror as the optical deflector, a second image forming optical systemthat allows a uniform spot with a constant speed to be formed on asurface to be scanned, a scanning starting signal detector that detectsa light beam scanned by the optical deflector, and a detecting opticalsystem that condenses a light beam from the light source onto thescanning starting signal detector.

The second image forming optical system in a conventional opticalscanner is configured of a so-called f-θ lens that is made up of aplurality of large-sized glass lenses, which has been a cause ofdifficulty in reducing size and high cost. With respect to this problem,in recent years, as disclosed in JP 8(1996)-94953 A and JP11(1999)-30710A, optical scanners for achieving size and cost reduction that use acurved surface mirror as the second image forming optical system havebeen proposed.

However, with regard to each of the above optical scanners that havebeen proposed, though it is explained that ideally, a light beam fromthe curved surface mirror is guided directly to an image plane, sincethe light beam is reflected off the curved surface mirror at a smallreflection angle, practically, in order for the light beam to be guidedto a photosensitive drum constituting a surface to be scanned, thefollowing configuration is required. That is, an optical path of anincreased length is provided, and a polygon mirror, the curved surfacemirror, and the photosensitive drum are disposed so that a distancebetween the polygon mirror and the curved surface mirror and a distancebetween the curved surface mirror and the photosensitive drum areincreased. Because of this, particular schemes have been required toreduce the size of the scanners.

DISCLOSURE OF THE INVENTION

In view of the above-mentioned problems, it is the first object of thepresent invention to provide an optical scanner that has a simpleconfiguration and allows effective use of its space so as to reduce thesize of the scanner, and an image forming apparatus. Further, the secondobject of the present invention is to provide an optical scanner thatincludes a curved surface mirror of a shape that provides relative easeof processing and measurement, and an image forming apparatus. Moreover,the third object of the present invention is to provide an opticalscanner that has excellent optical performance with no wavelengthdependency, and an image forming apparatus.

In order to achieve the above-mentioned objects, an optical scanneraccording to the present invention includes first and second reflectingoptical elements. In the optical scanner, the second reflecting opticalelement is disposed so as to reflect incident light to be incident onthe first reflecting optical element and reflected light from the firstreflecting optical element.

Furthermore, an image forming apparatus according to the presentinvention uses the above-mentioned optical scanner according to thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an optical scanner according toEmbodiment 1 of the present invention, which is cut on a plane thatincludes a scanning center axis and is parallel to a sub-scanningdirection.

FIG. 2 is a schematic plane view of the optical scanner according toEmbodiment 1 of the present invention.

FIG. 3 is a diagram for explaining arrangements in the optical scanneraccording to Embodiment 1 of the present invention.

FIG. 4 is a cross-sectional view showing a configuration of a curvedsurface mirror and its vicinity in the optical scanner according toEmbodiment 1 of the present invention, which is cut on the plane thatincludes the scanning center axis and is parallel to the sub-scanningdirection.

FIG. 5 is a schematic cross-sectional view of an image forming apparatusaccording to Embodiment 2 to which the optical scanner according to thepresent invention is applied.

FIG. 6 is a characteristic diagram showing spectral sensitivity of aphotodiode as a representative example.

FIG. 7 is a characteristic diagram showing spectral transmittances ofmaterials of three types of resin lenses as representative examples.

BEST MODE FOR CARRYING OUT THE INVENTION

According to the optical scanner of the present invention, the first andsecond reflecting optical elements are provided, and the secondreflecting optical element is disposed so as to reflect incident lightto be incident on the first reflecting optical element and reflectedlight from the first reflecting optical element, and thus a small-sizedoptical scanner can be obtained.

Furthermore, the image forming apparatus according to the presentinvention includes the above-mentioned optical scanner according to thepresent invention, and thus a small-sized image forming apparatus can beobtained.

Preferably, the above-mentioned optical scanner according to the presentinvention further includes a light source part, an optical deflectorthat scans a light beam from the light source part, and a first imageforming optical system that is disposed between the light source partand the optical deflector and allows a linear image to be formed on adeflection surface of the optical deflector, and has the followingconfiguration. That is, the first reflecting optical element is formedof a curved surface mirror, and is disposed between the opticaldeflector and a surface to be scanned and constitutes a second imageforming optical system. Further, the first image forming optical system,the optical deflector, and the second image forming optical system aredisposed respectively in different positions in a sub-scanning directionso that a light beam from the first image forming optical system isincident obliquely relative to a plane that includes a normal line tothe deflection surface of the optical deflector and is parallel to amain scanning direction, and so that a light beam from the opticaldeflector is incident obliquely relative to a plane that includes anormal line at a vertex of the curved surface mirror and is parallel tothe main scanning direction (hereinafter, referred to as a “YZ plane”).

Preferably, the first reflecting optical element is formed of a curvedsurface mirror, and the curved surface mirror has a shape symmetricalwith respect to a plane that includes the normal line at the vertex ofthe curved surface mirror and is perpendicular to the main scanningdirection (hereinafter, referred to as an “XZ plane”).

Preferably, the second reflecting optical element reflects the incidentlight to be incident on the first reflecting optical element and thereflected light from the first reflecting optical element by using acommon surface of the second reflecting optical element.

Herein, preferably, the first reflecting optical element is formed of acurved surface mirror, and when a distance between the first reflectingoptical element and the second reflecting optical element along thenormal line at the vertex of the curved surface mirror is indicated asl, an angle formed by a center axis of the incident light to be incidenton the first reflecting optical element and the YZ plane as θM, a widthof the incident light on the first reflecting optical element in thesub-scanning direction as dm, a width of the incident light to beincident on the first reflecting optical element on the secondreflecting optical element in the sub-scanning direction as di, and awidth of the reflected light from the first reflecting optical elementon the second reflecting optical element in the sub-scanning directionas do, the following conditional expression (1) is satisfied:

$\begin{matrix}{{\frac{dm}{2} + 1} \leqq {2l\mspace{11mu}\tan\mspace{11mu}\theta\; M} \leqq {\frac{di}{2} + \frac{do}{2} + 2}} & (1)\end{matrix}$

As an alternative, preferably, the first reflecting optical element isformed of a curved surface mirror, and when a distance between the firstreflecting optical element and the second reflecting optical elementalong the normal line at the vertex of the curved surface mirror isindicated as l, a distance between the first reflecting optical elementand the second reflecting optical element along the center axis of theincident light to be incident on the first reflecting optical element asLmi, a distance between the first reflecting optical element and thesecond reflecting optical element along a center axis of the reflectedlight from the first reflecting optical element as Lmo, an angle formedby the center axis of the incident light to be incident on the firstreflecting optical element and the YZ plane as θM, a width of theincident light on the first reflecting optical element in thesub-scanning direction as dm, a distance from the deflection surface ofthe optical deflector to the vertex of the first reflecting opticalelement as L, and a distance from the vertex of the first reflectingoptical element to the surface to be scanned as D, the followingconditional expression (1) is satisfied:

$\begin{matrix}{{\frac{dm}{2} + 1} \leqq {2l\mspace{11mu}\tan\mspace{11mu}\theta\; M} \leqq {\frac{di}{2} + \frac{do}{2} + 2}} & (1)\end{matrix}$where di and do are approximated by the following expressions (2) and(3), respectively:

$\begin{matrix}{{di} = {{dm} \times \frac{L - {Lmi}}{L}}} & (2) \\{{do} = {{dm} \times \frac{D - {Lmo}}{D}}} & (3)\end{matrix}$

Preferably, the first reflecting optical element is disposed in a spaceinterposed between incident light to be incident on the secondreflecting optical element and reflected light from the secondreflecting optical element.

Preferably, the above-mentioned optical scanner according to the presentinvention further includes a third reflecting optical element thatreflects the reflected light from the second reflecting optical element,and the first reflecting optical element is disposed in a spaceinterposed among the incident light to be incident on the secondreflecting optical element, the reflected light from the secondreflecting optical element, and reflected light from the thirdreflecting optical element.

Preferably, the first reflecting optical element has a shape thatpermits compensation for a curve of a scanning line that occurs due tooblique light incidence.

Preferably, the curved surface mirror has a skew shape in which a normalline at each of points other than the vertex on a generatrix that is acurved line where a surface of the curved surface mirror intersects withthe YZ plane is not included in the YZ plane.

Hereinafter, the present invention will be described by way ofembodiments with reference to the appended drawings.

EMBODIMENT 1

FIG. is a cross-sectional view showing an embodiment of the opticalscanner according to the present invention, which is cut on a plane thatincludes a scanning center axis and is parallel to a sub-scanningdirection. Further, FIG. 2 is a schematic plane view showing theembodiment of the optical scanner shown in FIG. 1. In FIG. 2, opticalelements such as a plane mirror and the like are represented by theirreflecting surfaces indicated by alternate long and short dashed linesfor the sake of simplicity.

In each of FIGS. 1 and 2, reference numeral 1 denotes a semiconductorlaser as a light source part. Further, reference numerals 2 and 3 denotean axial symmetric lens and a cylindrical lens that has a refractiveforce only in the sub-scanning direction, respectively, and the axialsymmetric lens 2 and the cylindrical lens 3 constitute a first imageforming optical system. Reference numerals 5 and 6 denote a polygonmirror as an optical deflector and a rotation center axis for thepolygon mirror 5, respectively. The polygon mirror 5 includes aplurality of deflection surfaces (reflecting surfaces) of the same shapearound the rotation center axis 6. Reference numerals 7 and 8 denote acurved surface mirror as a first reflecting optical element and aphotosensitive drum that is a surface to be scanned, respectively. Thecurved surface mirror 7 constitutes a second image forming opticalsystem. Reference numerals 51 and 52 denote a plane mirror as a secondreflecting optical element and a plane mirror as a third reflectingoptical element, respectively. The plane mirror 51 is disposed so as toreflect a light beam from the polygon mirror 5 toward the curved surfacemirror 7, and so as to reflect a reflected light beam from the curvedsurface mirror 7 toward the plane mirror 52. Further, the plane mirror52 is disposed so as to guide a light beam from the plane mirror 51 tothe photosensitive drum 8.

As shown in FIG. 1, the constituent components are disposed respectivelyin different positions with respect to the sub-scanning direction sothat a light beam from the semiconductor laser 1 passes through theaxial symmetric lens 2 and the cylindrical lens 3 to be incident on oneof the deflection surfaces of the polygon mirror 5 obliquely relative toa plane that includes a normal line to the one of the deflectionsurfaces and is parallel to a main scanning direction, and so that alight beam from the polygon mirror 5 is incident on the curved surfacemirror 7 obliquely relative to a YZ plane.

The curved surface mirror 7 is disposed in a space interposed betweenincident light L3 from the polygon mirror 5 that is to be incident onthe plane mirror 51 and reflected light L4 from the plane mirror 51 tothe plane mirror 52. Moreover, the curved surface mirror 7 is disposedin a space interposed among the incident light L3 to be incident on theplane mirror 51, the reflected light L4 from the plane mirror 51, andreflected light L5 from the plane mirror 52 to the photosensitive drum8.

Next, the description is directed to specific numerical examples.

First, parameters are defined as follows. As shown in FIG. 3, referencecharacter r denotes an inscribed radius of the polygon mirror 5.Further, reference character L denotes a distance between a deflectionreflection point on the polygon mirror 5 and a curved surface mirror 7′in the case where, as indicated by a chain double-dashed line, thecurved surface mirror 7 is disposed in the corresponding position of thecurved surface mirror 7′ shown in the figure on a light beam from thepolygon mirror 5 without using the plane mirror 51. Similarly, referencecharacter D denotes a distance between the curved surface mirror 7′ anda photosensitive drum 8′ in the case where the curved surface mirror 7and the photosensitive drum 8 are disposed in the respectivecorresponding positions of the curved surface mirror 7′ and thephotosensitive drum 8′ that are shown in the figure without using theplane mirrors 51 and 52. Reference character θP denotes an angle formedby a center axis of a light beam from the cylindrical lens 3 and anormal line to one of the deflection surfaces, and reference characterθM denotes an angle formed by a center axis of a light beam from one ofthe deflection surfaces and the YZ plane.

Furthermore, in this example, when with respect to a vertex of a surfaceof the curved surface mirror as an origin, a sag amount from the vertexat a position defined by a coordinate x (mm) in the sub-scanningdirection and a coordinate y (mm) in the main scanning direction isindicated as z (mm) where a direction in which an incident light beamtravels is defined as a positive direction, the surface has a shaperepresented by an expression (4):

$\begin{matrix}{Z = {{f(y)} + \frac{\frac{x^{2}}{g(y)} - {2{x \cdot \sin}\left\{ {\theta(y)} \right\}}}{{\cos\left\{ {\theta(y)} \right\}} + \sqrt{{\cos^{2}\left\{ {\theta(y)} \right\}} - \left( \frac{x}{g(y)} \right)^{2} + \frac{2{x \cdot \sin}\left\{ {\theta(y)} \right\}}{g(y)}}}}} & (4)\end{matrix}$

In the expression (4), f(y), g(y), and θ(y) are represented byexpressions (5), (6), and (7), respectively.

$\begin{matrix}{{f(y)} = {\frac{\left( \frac{y^{2}}{RDy} \right)}{1 + \sqrt{1 - {\left( {1 + k} \right)\left( \frac{y}{RDy} \right)^{2}}}} + {{AD} \cdot y^{4}} + {{AE} \cdot y^{6}} + {{AF} \cdot y^{8}} + {{AG} \cdot y^{10}}}} & (5) \\{{g(y)} = {{RDx}\left( {1 + {{BC} \cdot y^{2}} + {{BD} \cdot y^{4}} + {{BE} \cdot y^{6}} + {{BF} \cdot y^{8}} + {{BG} \cdot y^{10}}} \right)}} & (6) \\{{\theta(y)} = {{{EC} \cdot y^{2}} + {{ED} \cdot y^{4}} + {{EE} \cdot y^{6}}}} & (7)\end{matrix}$

Herein, f(y) is an expression representing a non-circular arc shape thatis a shape on a generatrix, g(y) is an expression representing a radiusof curvature at a y-position in the sub-scanning direction(x-direction), and θ(y) is an expression representing a skew amount at ay-position. Further, RDy (mm) denotes a radius of curvature in the mainscanning direction at the vertex, RDx (mm) denotes a radius of curvaturein the sub-scanning direction, and k denotes a cone constantrepresenting the shape of the generatrix. Further, AD, AE, AF and AGdenote high-order constants representing the shape of the generatrix,BC, BD, BE, BF, and BG denote constants determining a radius ofcurvature in the sub-scanning direction at a y-position, and EC, ED, andEE denote skew constants determining a skew amount at a y position.

Herein, the orders of y are all even numbers, which indicates that thecurved surface mirror 7 has a shape symmetrical with respect to an XZplane. Further, the curved surface mirror 7 has a skew shape in which anormal line at each of points other than the vertex on the generatrixthat is a curved line where the YZ plane intersects with the curvedsurface mirror 7 is not included in the YZ plane.

Specific numerical examples are shown in Tables 1 to 4 below. In thetables, a maximum image height is indicated as Ymax, and a polygonrotation angle corresponding to the maximum image height is indicated asamax.

NUMERICAL EXAMPLE 1

TABLE 1 Ymax   165 αmax    12.0 θP    5.0 θM    7.3 L   260.0 D   280.0r   12.5 RDy −798.091 RDx −270.256 K   0.00000E−00 BC  −2.1363E−06 EC−1.9803E−07 AD    1.3017E−10 BD  −1.4520E−12 ED −2.3053E−13 AE −2.9837E−16 BE    6.9318E−18 EE −1.4246E−18 AF   0.00000E−00 BF −6.6726E−23 AG   0.00000E−00 BG   0.00000E−00

NUMERICAL EXAMPLE 2

TABLE 2 Ymax   165 αmax    12.0 θP    5.0 θM    6.9 L   235.0 D   400.0r 12.5 Rdy −681.059 RDx −296.709 K   0.00000E−00 BC  −1.4968E−06 EC−2.0232E−07 AD    3.5335E−10 BD  −2.5755E−12 ED −6.7273E−14 AE −1.6918E−15 BE    1.2543E−17 EE −1.9834E−18 AF   0.00000E−00 BF −1.0406E−22 AG   0.00000E−00 BG   0.00000E−00

NUMERICAL EXAMPLE 3

TABLE 3 Ymax   165 αmax    12.0 θP    5.0 θM    7.4 L   250.0 D   300.0r 12.5 RDy −779.706 RDx −273.381 K 0.00000E−00 BC  −1.9766E−06 EC−2.0737E−07 AD  1.6939E−10 BD  −1.7436E−12 ED −1.8719E−13 AE  1.6344E−16BE    9.1385E−18 EE −1.2529E−18 AF 0.00000E−00 BF  −3.6269E−24 AG0.00000E−00 BG   0.00000E−00

NUMERICAL EXAMPLE 4

TABLE 4 Ymax   165 αmax    12.0 θP    5.0 θM     8.2 L   235.0 D   290.0 r  12.5 RDy −808.421 RDx  −260.376 K 0.00000E−00 BC −1.9268E−06 EC −2.4455E−07 AD  2.5557E−10 BD  −1.9637E−12 ED−1.2203E−13 AE  2.2322E−15 BE    2.4845E−17 EE −1.5950E−19 AF0.00000E−00 BF  −1.4253E−22 AG 0.00000E−00 BG   0.00000E−00

The following description is directed to the operation of each ofoptical scanners having the above-mentioned configurations withreference to FIGS. 1 to 3.

A light beam from the semiconductor laser 1 is converted into convergedlight by the axial symmetric lens 2. Then, the light beam is convergedonly in the sub-scanning direction by the cylindrical lens 3, and isfocused as a linear image on one of the deflection surfaces of thepolygon mirror 5. The polygon mirror 5 rotates in a direction indicatedby an arrow B around the rotation center axis 6, so that the light beamis scanned. Then, the light beam is reflected off the plane mirror 51,the curved surface mirror 7, the plane mirror 51, and the plane mirror52 in this order to form an image on the surface 8 to be scanned. Withrespect to the shape of the curved surface mirror 7, a non-circular arcshape in the cross section in the main scanning direction and a radiusof curvature in the sub-scanning direction corresponding to each imageheight are determined so that curvatures of field in the main andsub-scanning directions and an f-θ error are compensated, and moreover,an amount of a skew of the surface of the curved surface mirror 7 at aposition corresponding to each image height is determined so that acurvature of a scanning line is compensated. Further, a portion of thelight beam that has been deflected in a predetermined direction on aside slightly outer than a light beam scanning area on the surface 8 tobe scanned is focused on a photodiode that is not shown by the curvedsurface mirror 7. Using a detection signal from the photodiode as asynchronizing signal, a controller that is not shown controls thesemiconductor laser 1.

As described above, according to Embodiment 1 of the present invention,the plane mirror 51 is disposed so as to reflect incident light to beincident on the curved surface mirror 7 and reflected light from thecurved surface mirror 7. Thus, even in the case where a light beam isreflected off the curved surface mirror 7 at a small reflection angle, areduction of the size of a scanner can be achieved.

Furthermore, the second image forming optical system is configured ofone curved surface mirror 7 alone, and the first image forming opticalsystem, the polygon mirror 5, and the second image forming opticalsystem 7 are disposed respectively in different positions in thesub-scanning direction. Thus, a small-sized optical scanner that has asimple configuration and excellent optical performance with nowavelength dependency can be obtained.

Furthermore, the curved surface mirror 7 is formed in a shapesymmetrical with respect to the XZ plane. This allows the curved surfacemirror 7 to have a shape that provides relative ease of processing andmeasurement. Moreover, complete compatibility with the arrangementindicated by the chain double-dashed line shown in FIG. 3 can bemaintained. That is, in the case where the plane mirror 51 is placed onan optical path extending from the polygon mirror 5 toward the curvedsurface mirror 7′ indicated by the chain double-dashed line and on anoptical path extending from the curved surface mirror 7′ indicated bythe chain double-dashed line toward the plane mirror 52, simply bydisposing the curved surface mirror 7 in the position of a mirror imageof the curved surface mirror 7′ indicated by the chain double-dashedline, the arrangement shown in FIG. 1 can be obtained using the curvedsurface mirror 7 of the same shape as that of the curved surface mirror7′ without changing any other members. As a result of this, sizereduction of an optical scanner can be achieved.

Furthermore, the plane mirror 51 reflects incident light to be incidenton the curved surface mirror 7 and reflected light from the curvedsurface mirror 7 by using a common surface thereof, and thus asmall-sized optical scanner can be obtained using a minimum number ofcomponents.

Herein, as shown in FIG. 4, preferably, when a distance between thecurved surface mirror 7 and the plane mirror 51 along a normal line atthe vertex of the curved surface mirror 7 is indicated as l, an angleformed by a center axis of light to be incident on the curved surfacemirror 7 and the YZ plane as θM, a width of a light beam on the curvedsurface mirror 7 in the sub-scanning direction as dm (mm), a width ofthe light to be incident on the curved surface mirror 7 on the planemirror 51 in the sub-scanning direction as di (mm), and a width ofreflected light from the curved surface mirror 7 on the plane mirror 51in the sub-scanning direction as do (mm), the following conditionalexpression (1) is satisfied:

$\begin{matrix}{{\frac{dm}{2} + 1} \leqq {2l\mspace{11mu}\tan\mspace{11mu}\theta\; M} \leqq {\frac{di}{2} + \frac{do}{2} + 2}} & (1)\end{matrix}$

Alternatively, when a distance between the curved surface mirror 7 andthe plane mirror 51 along a center axis of light to be incident on thecurved surface mirror 7 is indicated as Lmi (mm), a distance between thecurved surface mirror 7 and the plane mirror 51 along a center axis ofreflected light from the curved surface mirror 7 as Lmo (mm), a distancefrom one of the deflection surfaces of the polygon mirror 5 to thevertex of the curved surface mirror 7 as L (mm), and a distance from thevertex of the curved surface mirror 7 to the photosensitive drum 8 as D(mm), di and do in the above-mentioned expression (1) may beapproximated by the following expressions (2) and (3), respectively:

$\begin{matrix}{{di} = {{dm} \times \frac{L - {Lmi}}{L}}} & (2) \\{{do} = {{dm} \times \frac{D - {Lmo}}{D}}} & (3)\end{matrix}$

With a value of 2l tan θM lower than the lower limit value given by theexpression (1), undesirably, the curved surface mirror 7 interrupts theincident light L3 or the reflected light L4. Further, with a value of 2ltan θM higher than the upper limit value given by the expression (1),undesirably, the plane mirror 51 has an increased width in thesub-scanning direction. In other words, when 2l tan θM has a value nothigher than the upper limit value given by the expression (1),significant advantages of the plane mirror 51 formed of one small-sizedmirror are attained. As a result, the number of components can bereduced, and adjustment of the optical systems can be facilitated.

Furthermore, the curved surface mirror 7 is disposed in a spaceinterposed between the incident light L3 to be incident on the planemirror 51 and the reflected light L4 from the plane mirror 51. Thus,space that has been considered useless in the case of the arrangementindicated by the chain double-dashed line in FIG. 3 can be utilizedeffectively, thereby allowing a small-sized optical scanner to beobtained.

Furthermore, the curved surface mirror 7 is disposed in a spaceinterposed among the incident light L3 to be incident on the planemirror 51, the reflected light L4 from the plane mirror 51, and thereflected light L5 from the plane mirror 52. Thus, space can be utilizedmore effectively, and a small-sized optical scanner can be obtained.

Moreover, by the above-mentioned configuration, the polygon mirror 5,the curved surface mirror 7, the plane mirror 51, and the plane mirror52 can be disposed closely to each other. Thus, as well as sizereduction of an optical scanner, higher rigidity of fixed parts of theabove-mentioned optical elements can be achieved, thereby allowing anoptical scanner that exhibits high stability with respect to a vibrationand a temperature change to be obtained.

Furthermore, the curved surface mirror 7 has a shape that permitscompensation of a curve of a scanning line that occurs due to obliquelight incidence. Thus, optical systems can be formed so as to havesimple configurations, and while ray aberration that occurs due tooblique incidence of a light beam can be compensated, a curve of ascanning line also can be compensated.

Furthermore, the curved surface mirror 7 has a skew shape in which anormal line at each of the points other than the vertex on thegeneratrix that is a curved line where the surface of the curved surfacemirror 7 intersects with the YZ plane is not included in the YZ plane.Thus, optical systems can be formed so as to have simple configurations,and while ray aberration that occurs due to oblique incidence of a lightbeam can be compensated, a curve of a scanning line also can becompensated.

Furthermore, this embodiment used the curved surface mirror 7 that isrepresented by the expression (4). Thus, even when the plane mirror 51vibrates, less influence is caused by the vibration, thereby allowing anexcellent image to be obtained.

In this embodiment, the expression (4) was used to represent the shapeof the curved surface mirror 7. However, other expressions also may beused as long as the expressions can represent the same shape.

Moreover, in the curved surface mirror 7, an angle formed by a normalline at each of the points on the generatrix and the YZ plane should beincreased toward the periphery of the curved surface mirror 7. Further,an angle formed by a normal line at each of the points on the generatrixwith respect to the YZ plane should be in a positive direction where adirection of an angle that a light beam reflected off the curved surfacemirror 7 forms with respect to an incident light beam from the polygonmirror 5 is defined as the positive direction.

Furthermore, in this embodiment, a light source for emitting a lightbeam having a wavelength of 500 nm or shorter can be used. FIG. 6 is acharacteristic diagram showing spectral sensitivity of a photodiode as arepresentative example. FIG. 7 is a characteristic diagram showingspectral transmittances of materials of three types of resin lenses asrepresentative examples. With respect to light beams in a region of awavelength of 500 nm or shorter, as shown in FIG. 6, the spectralsensitivity of the photodiode decreases to about half the value obtainedwith respect to a commonly used wavelength of 780 nm. Moreover, in thiscase, as shown in FIG. 7, when using a plurality of resin lenses, due tothe respective spectral transmittances of the lenses, the power of alight beam is attenuated, and thus the light beam hardly can be detectedby the photodiode. According to the optical scanner of the presentinvention, a detecting optical system that guides and condenses a lightbeam onto a photodiode is configured of one curved surface mirror 7alone. Thus, a reflectance of the mirror 7 as high as 95% or higher canbe achieved, thereby facilitating detection of a reference signal evenin the case of using a light source for emitting light having a shortwavelength.

EMBODIMENT 2

FIG. 5 is a schematic cross-sectional view showing an embodiment of animage forming apparatus to which the optical scanner described withregard to Embodiment 1 is applied. In FIG. 5, reference numeral 26denotes a photosensitive drum that includes a surface to be scannedcoated with a photosensitive material whose electric charge changesunder light irradiation, and reference numeral 27 denotes a primarycharger that allows electrostatic ions to adhere to a surface of thephotosensitive material so that the surface is charged. Further,reference numeral 28 denotes a developer that allows charged toner toadhere selectively to the photosensitive material, and reference numeral29 denotes a transferring charger that transfers the toner that has beenadhered thereto to a paper sheet. Reference numerals 30, 31, and 32denote a cleaner that removes a residue of the toner, a fixing unit thatfixes the transferred toner on the paper sheet, and a paper-feedingcassette, respectively. Reference numeral 33 denotes a light sourceblock that is composed of a semiconductor laser as a light source partand a first image forming optical system configured of an axialsymmetric lens and a cylindrical lens. Reference numerals 34, 35, 36,and 37 denote a polygon mirror as an optical deflector, a curved surfacemirror that is the first reflecting optical element described withregard to Embodiment 1, a plane mirror that is a second reflectingoptical element, and a plane mirror that is a third reflecting opticalelement, respectively.

As described above, according to Embodiment 2, the above-mentionedoptical scanner according to Embodiment 1 is used, and thus asmall-sized image forming apparatus can be realized even in the case ofusing a curved surface mirror as an optical element.

Furthermore, if a configuration in which a light beam from the lightsource block 33 is bent at a bending mirror (not shown) and then isincident on the polygon mirror 34 is employed, a further reduction insize can be achieved.

The embodiments disclosed in this application are intended to illustratethe technical aspects of the invention and not to limit the inventionthereto. The invention may be embodied in other forms without departingfrom the spirit and the scope of the invention as indicated by theappended claims and is to be broadly construed.

1. An optical scanner, comprising: first and second reflecting optical elements, wherein the second reflecting optical element is disposed so as to reflect incident light to be incident on the first reflecting optical element and reflected light from the first reflecting optical element, wherein the second reflecting optical element reflects the incident light to be incident on the first reflecting optical element and the reflected light from the first reflecting optical element by using a common surface of the second reflecting optical element, and wherein the first reflecting optical element is formed of a curved surface mirror, and when a distance between the first reflecting optical element and the second reflecting optical element along a normal line at a vertex of the curved surface mirror is indicated as l, an angle formed by a center axis of incident light to be incident on the first reflecting optical element and a plane that includes the normal line at the vertex of the curved surface mirror and is parallel to a main scanning direction as θM, a width of the incident light on the first reflecting optical element in a sub-scanning direction as dm, a width of the incident light to be incident an the first reflecting optical element on the second reflecting optical element in the sub-scanning direction as di, and a width of reflected light from the first reflecting optical element on the second reflecting optical element in the sub-scanning direction as do, the following conditional expression (1) is satisfied: $\begin{matrix} {{\frac{dm}{2} + 1} \leqq {2l\mspace{11mu}\tan\mspace{11mu}\theta\; M} \leqq {\frac{di}{2} + \frac{do}{2} + 2.}} & (1) \end{matrix}$
 2. The optical scanner according to claim 1, further comprising: a first image forming optical system that is disposed between the light source part and the optical deflector and allows a linear image to be formed on a deflection surface of the optical deflector, wherein the first reflecting optical element is formed of a curved surface mirror, and is disposed between the optical deflector and a surface to be scanned and constitutes a second image forming optical system, and the first image forming optical system, the optical deflector, and the second image forming optical system are disposed respectively in different positions in a sub-scanning direction so that a light beam from the first image forming optical system is incident obliquely relative to a plane that includes a normal line to the deflection surface of the optical deflector and is parallel to a main scanning direction, and so that a light beam from the optical deflector is incident obliquely relative to a plane tint includes a normal line at a vertex of the curved surface mirror and is parallel to the main scanning direction (hereinafter, referred to as a “YZ plane”).
 3. The optical scanner according to claim 1, wherein the first reflecting optical element is formed of a curved surface mirror, and the curved surface mirror has a shape symmetrical with respect to a plane that includes a normal line at a vertex of the curved surface mirror and is perpendicular to a main scanning direction (hereinafter, referred to as an “XZ plane”).
 4. The optical scanner according to claim 1, wherein the second reflecting optical element reflects a light beam reflected from the optical deflector and a light beam reflected from the first reflecting optical element by using a common surface of the second reflecting optical element.
 5. The optical scanner according to claim 2, wherein the flint reflecting optical element has a shape that permits compensation for a curve of a scanning line that occurs due to oblique light incidence.
 6. The optical scanner according to claim 2, wherein the curved surface mirror has a skew shape in which a normal line at each of points other than die vertex on a generatrix that is a curved line where a surface of the curved surface mirror intersects with the YZ plane is not included in the YZ plane.
 7. The optical scanner according to claim 1, wherein the first reflecting optical element is disposed in a space interposed between a light beam reflected from the optical deflector to be incident on the second reflecting optical element and a light beam reflected last from the second reflecting optical element.
 8. The optical scanner according to claim 1, further comprising: a third reflecting optical element that reflects a light beam reflected last from the second reflecting optical element, wherein the first reflecting optical element is disposed in a space bounded by the second reflecting optical element, a light beam reflected from the optical deflector to be incident on the second reflecting optical element, the light bean reflected last from the second reflecting optical element to be incident on the third reflecting optical element, and a light beam reflected from the third reflecting optical element.
 9. An image forming apparatus comprising an optical scanner as claimed in claim
 1. 10. An optical scanner, comprising: first and second reflecting optical elements, wherein the second reflecting optical element is disposed so as to reflect incident light to be incident on the first reflecting optical element and reflected light from the first reflecting optical element, wherein the second reflecting optical element reflects the incident light to be incident on the first reflecting optical element and the reflected light from the first reflecting optical element by using a common surface of the second reflecting optical element, and wherein the first reflecting optical element is formed of a curved surface mirror, and when a distance between the first reflecting optical element and the second reflecting optical element along a normal line at a vertex of the curved surface mirror is indicated as l, a distance between the first reflecting optical element and the second reflecting optical element along a center axis of incident light to be incident on the first reflecting optical element as Lmi, a distance between the first reflecting optical element and the second reflecting optical element along a center axis of reflected light from the first reflecting optical element as Lmo, an angle formed by the center axis of the incident light to be incident on the first reflecting optical element and a plane that includes the normal line at the vertex of the curved surface mirror and is parallel to a main scanning direction as θM, a width of the incident light on the first reflecting optical element in a sub-scanning direction as dm, a distance from a deflection surface of the optical deflector to a vertex of the first reflecting optical element as L, and a distance from the vertex of′ the first reflecting optical element to a surface to be scanned as D, the following conditional expression (1) is satisfied: $\begin{matrix} {{\frac{dm}{2} + 1} \leqq {2l\mspace{11mu}\tan\mspace{11mu}\theta\; M} \leqq {\frac{di}{2} + \frac{do}{2} + 2}} & (1) \end{matrix}$ where di and do are approximated by the following expressions (2) and (3), respectively: $\begin{matrix} {{di} = {{dm} \times \frac{L - {Lmi}}{L}}} & (2) \\ {{do} = {{dm} \times {\frac{D - {Lmo}}{D}.}}} & (3) \end{matrix}$
 11. The optical scanner according to claim 10, further comprising: a first image forming optical system that is disposed between the light source part and the optical deflector and allows a linear image to be formed on a deflection surface of the optical deflector, wherein the first reflecting optical element is formed of a curved surface mirror, and is disposed between the optical deflector and a surface to be scanned and constitutes a second image forming optical system, and the first image forming optical system, the optical deflector, and the second image forming optical system are disposed respectively in different positions in a sub-scanning direction so that a light beam from the first image forming optical system is incident obliquely relative to a plane that includes a normal line to the deflection surface of the optical deflector and is parallel to a main scanning direction, and so that a light beam from the optical deflector is incident obliquely relative to a plane that includes a normal line at a vertex of the curved surface mirror and is parallel to the main scanning direction (hereinafter, referred to as a “YZ plane”).
 12. The optical scanner according to claim 11, wherein the first reflecting optical element has a shape that permits compensation for a curve of a scanning line that occurs due to oblique light incidence.
 13. The optical scanner according to claim 11, wherein the curved surface mirror has a skew shape in which a normal line at each of points other than the vertex on a generatrix that is a curved line where a surface of the curved surface mirror intersects with the YZ plane is not included in the YZ plane.
 14. The optical scanner according to claim 10, wherein the first reflecting optical element is formed of a curved surface mirror, and the curved surface mirror has a shape symmetrical with respect to a plane that includes a normal line at a vertex of the curved surface mirror and is perpendicular to a main scanning direction (hereinafter, referred to as an “XZ plane”).
 15. The optical scanner according to claim 10, wherein the second reflecting optical element reflects a light beam reflected from the optical deflector and a light beam reflected from the first reflecting optical element by using a common surface of the second reflecting optical element.
 16. The optical scanner according to claim 10, wherein the first reflecting optical element is disposed in a space interposed between a light beam reflected from the optical deflector to be incident on the second reflecting optical element and a light beam reflected last from the second reflecting optical element.
 17. The optical scanner according to claim 10, further comprising: a third reflecting optical element that reflects a light beam reflected last from the second reflecting optical element, wherein the first reflecting optical element is disposed in a space bounded by the second reflecting optical element, a light beam reflected from the optical deflector to be incident on the second reflecting optical element, the light beam reflected last from the second reflecting optical element to be incident on the third reflecting optical element, and a light beam reflected from the third reflecting optical element.
 18. An image forming apparatus comprising an optical scanner as claimed in claim
 10. 